1. Field of the Invention
The present invention relates to a bidirectional signal transmission device using light, which is provided to convert an electrical signal into an optical signal, transmit the optical signal, and convert the transmitted optical signal back into an electrical signal, so that bidirectional signal transmission and reception between two apparatuses are achieved, and more particularly, to a bidirectional signal transmission device using light which employs a photodiode-incorporated vertical cavity surface emitting laser (VCSEL) as a light device so that an optical signal is transmitted and received via the same channel.
2. Description of the Related Art
Electrical signals are typically transmitted in one direction along an electrical wire. However, in many cases, bidirectional transmission of electrical signals via one or two electrical wires is required. For example, an I2C serial bus or a universal serial bus (USB) line, which is used to facilitate data exchange between a desktop monitor and a video graphics card, requires bidirectional transmission of an electrical signal.
Bidirectional signal transmission using an electrical wire cannot provide transmission over a long distance due to a limit in the transmission speed, and may cause electromagnetic interference or the like. Hence, an optical signal transmission method enabling high-speed transmission without electromagnetic interference has been adopted to achieve bidirectional signal transmission.
Generally, for bidirectional optical signal transmission, an optical fiber serving as a transmission channel and an optical fiber serving as a reception channel are separately required, and reception and transmission of an optical signal are performed individually. However, the separate installation of a transmission channel and a reception channel complicates the hardware structure of a signal transmission device and increases the manufacturing costs thereof, so that this optical signal transmission method is difficult to put into use in spite of its numerous advantages.
In the prior art, a bidirectional signal transmission device designed to bidirectionally transmit an optical signal via the same channel, as shown in FIG. 1, has been proposed in consideration of the above-described point.
Referring to FIG. 1, the conventional bidirectional signal transmission device includes first and second light devices 1 and 5 for emitting light and receiving incident light, and an optical fiber 4 installed so that its input/output ports face the optical devices. The first and second light devices 1 and 5 include first and second light receiving elements 2 and 6 for receiving incident light, respectively, and first and second VCSELs 3 and 7 die-bonded to the centers of the first and second light receiving devices 2 and 6, respectively.
When an electrical signal from an apparatus (not shown) is received by the first optical device 1, the first VCSEL 3 emits an optical signal in response to the received electrical signal, and the optical signal is transmitted to the second light receiving element 6 via the optical fiber 4. Then, the second light receiving element 6 outputs an electrical signal that is proportional to the amount of received light, and the electrical signal is transmitted to another apparatus (not shown). Signal transmission in a direction opposite to the above-described signal transmission is achieved via the optical fiber 4 using the second VCSEL 7 and the first light receiving element 2 by the same method as the method described above.
The conventional bidirectional signal transmission device of FIG. 1 uses the single optical fiber 4 as a transmission channel and a reception channel.
However, in a conventional bidirectional signal transmission device, as shown in FIG. 1, light to be emitted from the input/output port of the optical fiber 4 is typically concentrated at the center of the optical fiber, so that the light receiving-regions 2a and 6a of the light receiving elements, that are at the outside of each VCSEL, receive small amounts of light. Therefore, a received signal has a low signal-to-noise (SIN) ratio.
Also, when signal transmission is performed in two directions simultaneously, the light receiving elements receive not only light transmitted from the optical fiber 4 but also light which is emitted from the VCSELs on the light receiving elements and partially reflected by the input/output port of the optical fiber 4. Thus, there is crosstalk on a detection signal, so that, the S/N ratio of a received signal is greatly degraded.
Also, a VCSEL receives part of the light that is emitted from a VCSEL on the opposite side and transmitted via the optical fiber 4, so that the relative intensity noise of the VCSEL is degraded. Thus, when transmission and reception of a signal are performed simultaneously in dual time at high speed, the bit error rate characteristic deteriorates.
Furthermore, the conventional bidirectional signal transmission device has a structure in which the VCSELs 3 and 7 are die-bonded to the light receiving elements 2 and 6, respectively, so that a yield reduction due to assembly error and bonding error may be caused during die bonding.
To solve the above problems, an objective of the present invention is to provide a bidirectional signal transmission device using light, which adopts as a light device a VCSEL incorporated into a photodiode in a semiconductor manufacturing process, and is designed to increase the light receiving efficiency by installing a diffraction element for diffracting light emitted from an optical fiber and directing the diffracted light to the light receiving region of a light receiving element, between the light device and the input/output port of the optical fiber.
The above objective of the present invention is achieved by a bidirectional signal transmission device using light including: an optical unit including at least one of first and second light devices having a vertical cavity surface emitting laser (VCSEL) unit for emitting light in the direction semiconductor material layers are stacked and a photodiode part incorporated with the VCSEL part for receiving incident light, at least one optical fiber installed between facing first and second light devices for transmitting an optical signal, and diffraction elements installed between the first light device and one input/output port of the optical fiber, and between the second light device and the other input/output port of the optical fiber, respectively; for selectively diffracting incident light so that light output from the optical fiber is received by the photodiode part; a first circuit unit for controlling the first light device, so that an electrical signal from a first apparatus is converted into an optical signal, and so that an optical signal transmitted via the optical fiber is converted into an electrical signal and then transmitted to the first apparatus; and a second circuit unit for controlling the second light device so that an electrical signal from a second apparatus is converted into an optical signal, and so that an optical signal transmitted via the optical fiber is converted into an electrical signal and then transmitted to the second apparatus, wherein signal transmission and reception between the facing first and second light devices are performed via the same channel.
According to an aspect of the present invention, the diffraction element is a diffraction grating or a hologram optical element for diffracting incident light in a +1st and/or xe2x88x921st order.
According to another aspect of the present invention, the corresponding first and second light devices emit perpendicular polarized light beams, and the diffraction element is a polarization diffraction element designed so that polarized light incident from the VCSEL part is transmitted straight through the diffraction element and received by the optical fiber, and so that polarized light incident from the optical fiber is diffracted in a +1st and/or xe2x88x921st order and headed for the photodiode part.
Here, the optic axis of the polarization diffraction element is approximately perpendicular to the direction of linear polarization of light emitted from a VCSEL part that faces the polarization diffraction element.
Each of the first and second circuit units includes: a driver for driving a VCSEL part; and an amplifier for amplifying a current signal output from the photodiode part.
Preferably, each of the first and second circuit units further includes a controller installed between an apparatus and the amplifier and the driver, for converting an electrical signal from the apparatus into a signal that is suitable for controlling the driver, and converting a signal from the amplifier into a signal suitable for the apparatus.
It is also preferable that the first and/or second circuit unit further includes a switch installed between the controller and the driver and amplifier, for selectively switching on and off the driver and/or the amplifier.